The Effect of a Casing Step on the Over-Tip Aerothermodynamics of a Transonic HP Turbine Stage

The casing end-wall of a shroudless high-pressure turbine is subject to severe thermal loading. A backwards facing step positioned upstream of the leading edge of the rotor has been shown to reduce the heat flux in this region. The testing has been conducted at fully engine scaled Mach number, Reynolds number and gas-to-wall temperature ratio. Measurements of local heat-flux and static pressure were made using thin-film and semi-conductor gauges respectively. Normalised heat-flux, static pressure, adiabatic wall temperature and heat-transfer coefficient distributions are compared with those from a previously tested plain annulus baseline geometry. The introduction of the casing step has lowered the casing end-wall heat-load. This reduction is associated primarily with a decrease in adiabatic wall temperature on the casing end-wall. Analysis of the time-resolved static pressure distribution, combined with a Computational Fluid Dynamics simulation, has shown that this reduction in adiabatic wall temperature is the result of work extraction from the fluid entrained behind the casing step. The variation of the blade tip loading with upstream guide vane phase has also been reduced.